The present invention generally relates to semiconductor laser devices. More particularly, the present invention relates to a semiconductor laser device that is preferably used as a component of an optical pickup for use in an apparatus for reading from and/or writing to a plurality of optical disks, and a method of manufacturing such semiconductor laser devices.
An optical pickup that emits light by utilizing a semiconductor laser is used as a light source for reading and/or writing of optical disks. Different kinds of optical disks require different optimum semiconductor laser wavelengths. The wavelengths optimum to the CD, the CD-R, and the DVD are a wavelength less than 800 nm, a wavelength in the neighborhood of 780 nm, and 650 nm, respectively.
Accordingly, an apparatus compatible with a plurality of kinds of optical disks such as the CD and the DVD is equipped with a plurality of different optical pickups, and switches between those pickups according to the type of a loaded disk. FIG. 1 shows an example of such an apparatus.
In the example shown in FIG. 1, there are two optical pickups P1, P2 that emit light having different wavelengths. One of these optical pickups is selected depending on the kind of an optical disk D loaded. The optical pickups P1, P2 include semiconductor lasers L1, L2 and collimator lens C1, C2 respectively. An optical system S including a half mirror is disposed between an objective lens O and each of the optical pickups P1, P2. The optical system S allows light emitted by both optical pickups P1, P2 to reach the optical disk D through the objective lens O. Light reflected by the optical disk D is read by an unshown photosensor provided in each of the optical pickups P1, P2.
In the example of FIG. 1, the two optical pickups P1, P2, which are separately disposed, occupies a large space. Further, because the lights emitted by respective optical pickups are required to reach the optical disk D, a complicated design of the optical system is required.
The present invention has been made to effectively solve the above problems of the conventional art. The present invention realizes production of an optical pickup capable of emitting lights having different wavelengths. The present invention provides a semiconductor laser device having the following characteristic features.
The semiconductor laser device of the present invention comprises a stem having a mounting surface, and first and second semiconductor laser elements directly or indirectly mounted onto the mounting surface of the stem, these two semiconductor laser elements having different emission wavelengths and different temperature dependences. The first semiconductor laser element that has a higher temperature dependence is located closer to the mounting surface of the stem. The second semiconductor laser element having lower temperature dependence is disposed on top of the first semiconductor laser element having higher temperature dependence. The first semiconductor laser element may be disposed on the stem directly or through an insulative submount, depending on a wiring condition or the like.
In the semiconductor laser device having the construction, lights having different wavelengths can be selectively emitted by the stacked first and second laser elements. Thus, an optical pickup using the semiconductor laser device emits two kinds of light beams selectively. Thus, an optical pickup that is made using the semiconductor laser device of the present invention is adaptable to two kinds of optical disks without increasing the size of an apparatus in which the optical pickup is included.
The reason why the first laser element having a higher temperature dependence is disposed closer to the mounting surface of the stem is as follows. Heat generated by the laser elements is released to the outside through the stem. This means that heat generated by the first laser element disposed closer to the mounting surface of the stem is easy to dissipate, while heat generated by the second laser element farther to the mounting surface of the stem is hard to dissipate. Accordingly, it is advisable to dispose a laser element less susceptible in an upper position far from the mounting surface of the stem. Conversely, a disadvantage would not occur if the laser element having a relatively high temperature dependence is disposed adjacently to the mounting surface of the stem.
In the present invention, light emitted by the first laser element having a higher temperature dependence may have an emission wavelength, for example, in the range of 640-660 nm, while light emitted by the second laser element having a lower temperature dependence may have an emission wavelength, for example, in the range of 770-800 nm. In this case, the semiconductor laser device is adaptable both to the DVD (by the provision of the laser element of 640-660 nm in wavelength) and to the CD or the CD-R (by the provision of the other laser element of 770-800 nm in wavelength).
In the semiconductor laser device of the present invention, the second semiconductor laser element which is disposed on the first semiconductor laser element may, preferably, be smaller in size than the first semiconductor laser element such that a part of a top surface of the first semiconductor laser element is exposed. This construction facilitates a wiring operation for a common electrode formed on an upper surface of the first laser element.
In the semiconductor laser device of the present invention, the first and second semiconductor laser elements may, preferably, have their P-layers or N-layers disposed adjacent to each other. By adopting the construction, it is possible to construct a common electrode between the two laser elements as an anode-common type or a cathode-common type, resulting in the simplification of a driving circuit.
In the semiconductor laser device of the present invention in which two laser elements are disposed one on top of the other, emission points of the two laser element may preferably be located at an interval as short as possible, in order to obtain preferable collimated light by making light emitted from each emission point incident on one collimator lens. More specifically, the interval between the two emission points may preferably be 160 micrometers or less.
As a means for realizing the above interval between the two emission points, it is conceivable to form each laser element to a height of 160 micrometers or less and locate the emission point at a center of the height of each laser element. Alternatively, it is possible to set the height of each laser element to 80 micrometers or less. In this case, because a total of the heights of both laser elements is 160 micrometers or less, the interval between both emission points necessarily becomes 160 micrometers or less, irrespective of the positions of the emission points.
In the present invention, it is possible to stack three or more laser chips one on another on or above the mounting surface of the stem, the three or more laser chips having different emission wavelengths and different temperature dependences. In this case, these semiconductor laser elements are stacked in order of temperature dependence such that the laser chip farther from the mounting surface of the stem has a lower temperature dependence than the laser chip closer to the mounting surface of the stem.
In fabricating the semiconductor laser device, it is necessary to join the laser elements to the stem or submount as well as to each other. The joint may be achieved by using conductive resins or soldering materials. When using soldering materials, it is preferable that soldering materials to be applied to different joined portions have different melting points, as described below. That is, a soldering material having a lower melting point is applied to a joined portion for which a joining operation is performed later. In other words, a soldering material having a highest melting point is used in an initial joining operation and a soldering material having a lower melting point is used in a later joining operation. In this way, it is possible to effectively prevent occurrence of a disadvantage that a soldering material at a previously joined portion melts in a subsequent joining operation.